The pulse is a very important parameter that is used to aid users of automated external defibrillators in determining whether or not to administer a defibrillation shock to and/or to perform cardiopulmonary resuscitation (CPR) on a victim who appears to be in cardiac arrest. Such a victim may actually be in need of cardiac resuscitation (including defibrillation and/or CPR), or may be suffering from a condition for which such treatment would be unsuitable, e.g., a stroke, seizure, diabetic coma, or heat exhaustion. It is very important to the safety of the victim that the presence or absence of a pulse be determined quickly and accurately. However, it is often difficult for trained medical personnel to take a victim's pulse accurately in the field during a crisis situation, and may be impossible for a minimally trained or untrained lay rescuer to do so. In many cases, it will take the person assisting the victim a considerable time (on the order of one minute or more) to find the victim's pulse. If a pulse is not found, the caregiver is left unsure as to whether the victim does not have a pulse, or whether the caregiver simply cannot find the victim's pulse.
Another parameter that is used in determining whether to administer a defibrillation shock is an ECG analysis of the victim's heart rhythm that is provided by the automated external defibrillator. Based on the ECG analysis, many automated defibrillators will provide the user with a message indicating whether a shock should be administered (i.e., whether or not ventricular fibrillation is present).
Generally, the ECG analysis systems in most commercially available automated external defibrillators display only two options to the user: “Shock Advised” or “No Shock Advised.” When “Shock Advised” is output, this means that the patient is in ventricular fibrillation or wide complex ventricular tachycardia above 150 BPM, conditions which are effectively treated by defibrillation. When “No Shock Advised” is output, this means that the patient's heart rhythm is not treatable by defibrillation therapy.
If the message indicates that a shock is not appropriate, this does not necessarily mean that the victim is not in danger. There are two ECG rhythms, generally referred to as asystole and pulseless electrical activity, which should not be treated with defibrillation (and thus will trigger a message not to shock) but nonetheless are extremely serious in that they suggest that the patient's heart rhythm is unaccompanied by sufficient cardiac output (i.e., the patient is close to death). These conditions are treated by administering cardiopulmonary resuscitation (CPR), in an effort to provide blood flow to the heart and vital organs in the hope that with improved blood flow and oxygenation, the heart muscle will recover from its near death state and possibly begin to fibrillate again, thus making defibrillation treatment a viable option.
Thus, when a “No Shock Advised” analysis is output, the caregiver does not know whether this result is caused by a normal heart rhythm, an abnormal but perfusing heart rhythm (i.e., the patient was never in cardiac arrest or the last shock treatment returned the patient's heart rhythm to normal), or a grossly abnormal (non-perfusing) ECG rhythm requiring CPR treatment. Because of this uncertainty, the normal medical protocol when “No Shock Advised” is output is to check the patient for a pulse and if no pulse is detected to start CPR. If a pulse is detected, then the patient's heart is effectively pumping blood and neither CPR nor defibrillation is warranted. If the victim does not have a pulse, CPR should be started immediately; if a pulse is present, then CPR should not be administered. Because CPR, even if properly administered, can result in broken ribs or other injury to the victim, it is undesirable to administer CPR if it is not actually necessary. Thus, it is again vitally important that an accurate determination of the presence or absence of a pulse be made by the caregiver.
A similar situation of uncertainty occurs after the third defibrillation shock is delivered in the three-shock protocol recommended by the American Heart Association. In this case, if the patient's fibrillation has not been “cured” after delivery of three shocks, the caregiver is instructed to perform CPR on the patient. Because automated external defibrillators generally do not perform an ECG analysis immediately after the third shock, the caregiver does not know whether the third shock provided effective treatment. Therefore, the caregiver must determine whether the patient has a pulse in order to determine whether CPR is needed or whether the patient is out of danger.
A wide variety of sensors have been employed for pulse detection.
Optical sensors have been used in pulse detection. For example, pulse detectors of the type used for measuring heart rate during exercise typically rely on reflectance or transmission of an infrared light beam. Blood pulsing in the user's capillaries produces a corresponding variation in the absorption of light by capillaries, and that variation produces a pulsation in the output of an optical sensor.
Optical sensors are also widely used in pulse oximetry, in which a measurement is made of the percentage of hemoglobin saturated with oxygen. An optical plethysmographic probe attached to the patient's finger or ear lobe generates light at two wavelengths (e.g., 650 nm and 805 nm). The light is partially absorbed by hemoglobin, by amounts that differ depending on whether or not the hemoglobin is saturated with oxygen. By calculating absorption at the two wavelengths, a pulse oximetry device can compute the proportion of hemoglobin that is saturated (oxygenated).
Acoustic sensors have also been applied to pulse detection. Typically, the acoustic sensor is configured to detect sounds characteristic of a beating heart (e.g., the action of a heart valve). E.g., Joo U.S. Pat. No. 6,440,082. Some acoustic sensors used for pulse detection are based on piezoelectric devices.
Another use of a piezoelectric devices in pulse detection is proposed in U.S. patent application Ser. No. 9/846,673, filed on May 1, 2001. The piezoelectric device detects motion of the surface of the body resulting from the pulse (e.g., motion resulting from blood flowing in a blood vessel beneath the sensor).